Holography is a form of optical information storage. The general principles are described in a number of references, e.g., “Photography by Laser” by E. N. Leith and J. Upatnieks in SCIENTIFIC AMERICAN, 212, No. 6, 24-35 (June, 1965). In brief, the object to be photographed or imaged is illuminated with collimated light, e.g., from a laser, and a light sensitive recording medium, e.g., a photographic plate, is positioned so as to receive light reflected from the object. Each point on the object reflects light to the entire recording medium, and each point on the medium receives light from the entire object. This beam of reflected light is known as the object beam. At the same time, a portion of the collimated light is beamed by a mirror directly to the medium, by passing the object. This beam is known as the reference beam. What is recorded on the recording medium is the interference pattern that results from the interaction of the reference beam and the object beam impinging on the medium. When the processed recording medium is subsequently illuminated and observed appropriately, the light from the illuminating source is diffracted by the hologram to reproduce the wave-front that originally reached the medium from the object, so that the hologram resembles a window through which the virtual image of the object is observed in full three-dimensional form, complete with parallax.
Holograms that are formed by allowing the reference and object beams to enter the recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are normal or near normal to the plane of the recording medium. When the hologram is played back by viewing with transmitted light, these fringes diffract the light to produce the viewed virtual image. Such transmission holograms may be produced by methods which are well known in the art, such as those disclosed in U.S. Pat. No. 3,506,327; U.S. Pat. No. 3,838,903 and U.S. Pat. No. 3,894,787, each of which is incorporated herein by reference.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides, so that they are traveling in approximately opposite directions, are known as reflection holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, planes parallel to the plane of the recording medium. When the hologram is played back these fringes act as mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection rather than in transmission. Since the wavelength sensitivity of this type of hologram is very high, white light may be used for reconstruction. Reflection holograms produced by an off-axis process are disclosed in U.S. Pat. No. 3,532,406, which is incorporated herein by reference.
More and more, holograms as described above are being used as an enhanced security means attached to commercial products, such as digital optical disks, compact disks, batteries for electronic products, and any other product that may be susceptible to counterfeiting efforts. The use of a hologram for identification purposes on such products is known. In the majority of prior art references, such a hologram is a surface-relief hologram formed by a stamping process. This process may be incorporated into the manufacturing process of the product. There are disclosures of holograms (volume-phase hologram) being formed and then being applied to a product by way of a label. While using the hologram is advantageous for security devices, there is a significant drawback to this approach in that the hologram can be counterfeited and applied to non-authentic counterfeited products. Therefore, such a holographic stamp or label has limited value as a security device.
There is a significant need for a security device that offers a higher level of security than described above. The present invention provides a solution to this important need.